Such system requires remote-control transponder installed with subminiature transmitter-receiver circuits capable of being carried in any moving objects therein. Particularly, the present invention provides a passive transponder to identification technique at relatively long range and high-speed driving objects.
Conventional transponder identification system transmits a single radio frequency to detect by antenna such as transponder tag, and non-linear impedance of semiconductor diode used for this generates selectively high frequency(“HF”) of transmitted signals which is re-radiated to be detected by receiving device. Such identification system has a shortage of practically not used due to occurrence of reading error at responding to sensitivity and different conditions to ensure detection of transponder within detection area.
It is importantly noticed that the transmitter-receiver circuit and element thereof are non-linear and generate HF signal as well as fundamental transmitting frequency and results the signals input into the receiver, even for transponder without non-linear impedance element. If sensitivity of the receiver is lowered to reduce the effect of harmonics directly radiated, weak-energy harmonics re-radiated by the passive transponder element will be absorbed in the former and not reach to the receiver. Of course, such problem may be minimized by ideally shielding the transmitter and the receiver and/or by means of RF Filtering circuit. However, the filter has a problem of causing even small frequency shift of the transmitted signals to easily serve as re-radiation frequency present outside of the filter pass-band, thus requiring very sophisticated shielding capability. Furthermore, frequency shift may be resulted from Doppler effect occurred in the transponder moving at high speed within the detection area. While these harmonic signals easily transmitting to the outside cause undesired data information to be radiated from any transponder beyond the detection area.
Consequently, HF energy which transmits by unexpected reflection such as multi-stationary wave is not effective to accomplish the original purpose of the system.
On the contrary, if insufficient amount of energy is received or re-radiated by harmonics, the system may not respond even the transponder element being actually present, within the detection area. For example, such condition may happen when antenna of the transponder antenna is unsuitably orientating corresponding to transmission field or is electro-magnetically blocked from receiver by nearby vehicles, partition wall on road, interception bar of toll gate and so on. Also, in case the transponder coming close to human body or metallic objects, it deflects tuning of a resonant tank circuit to result the dissipation of HF energy required by receiver. Although the transponder may be constructed to control frequency response from the receiver so to compensate frequency alteration by means of signal tracking circuit, the work efficiency of the transponder is decreased whenever the tuning tank circuit is run at any frequency other than normal one.
Alternatively, as a solution to overcome the problem to restrict HF transmission within the detection area in which non-linear impedance element acting as a signal mixer to generate summed or subtracted frequency between both transmitting signals having different frequencies each other is present, double field system using low frequency (“LF”) electro-statistic field formed between discontinuous conductors arranged opposite to the HF electro-magnetic field and the detection area is adaptable. The non-linear impedance element applied to such two fields serves often as a mixer to generate summed or subtracted frequency to be re-radiated to the receiver.
However, even though power consumed in the detection area is important to form electro-statistic field required within the detection area, such field may be unfortunately prevented from the transponder by conductor enclosing it or conversed from the transponder by traffic signs made of metallic materials or other metal structures around.
In addition, LF field is likely to be under reading error or immobile condition at long distance by nearby metal structures.